It is known practice to detect the current angle of rotation of a rotating body with the aid of magnetic Hall elements which are arranged along a circle circumference whose centre point is at right angles to the axis of rotation. For this purpose, the body is assigned a magnetic field, for example by connecting it to a magnet whose field strength varies at the sensor location on the basis of the sine of the angle of rotation. In this case, the Hall elements generate an electrical voltage, the so-called Hall voltage, which is both proportional to the magnetic field and to the bias current of the Hall elements, in accordance with the following formula for the Hall effect:Vh=B×I×Rh,where Vh is the Hall voltage, B is the magnetic field, I is the bias current and Rh is a constant for the Hall sensor. Hall sensors are usually operated using identical or matched constant bias currents. In order to detect the magnetic field of a simple, two-pole magnet, use may be made of two Hall sensors which provide two differential signals and are dependent, for example, on the sine or cosine of the angle of rotation to be determined.
A sensor arrangement having Hall elements is disclosed, for example, in U.S. Pat. No. 7,095,228. The individual sensors in the sensor arrangement are connected to signal modulators which make it possible to combine the individual sensor signals, the parameter to be detected being determined from all of the plurality of individual signals.
In typical applications, the sensor signals obtained (Hall voltages) are amplified. In order to accurately determine the angle or obtain higher resolution when determining the angle, interpolation is usually carried out. Said interpolation may be carried out in an IC, whose function is similar to that of an analogue/digital converter, by converting the analogue sine and cosine signals into analogue or digital signals which are at a higher angular frequency than that of the rotating body to be determined. One known sensor has, for example, a ten-bit output which requires 256-fold interpolation.
Different methods can be used for interpolation. A first possibility is to use conventional analogue/digital converters to convert the sensor signals into digital values which are proportional to the corresponding sensor signal, that is to say proportional to the sine or cosine value of the angle of rotation. The digital values can then be processed in digital function tables, for example CORDIC, and provide digital signals which are proportional to the angle of rotation to be determined.
A second method uses the sensor signals and directly converts them into digital values, a specially designed analogue/digital converter having the correct trigonometric characteristic being used. Digital values which are directly proportional to the angle to be determined are obtained as the result.
According to a third known method, the sensor signals are processed in the analogue domain in order to again obtain an analogue signal at a multiplied angular frequency. This may be carried out, for example, using a symmetrical mixer circuit, as is conventional in wireless communication terminals. The principle of such a symmetrical mixer is determined by the two following equations:sin 2θ=2 sin θ cos θcos 2θ=cos2θ−sin2θ,where sin θ and cos θ are the sensor signals and sin 2θ and cos 2θ are the useful signals which are obtained at the mixer output and are at twice the angular frequency in comparison with the rotation of the body. It is clear that the function and quality of a symmetrical mixer depend on the ability of the mixer to multiply, add and subtract in the analogue domain. A mixer for wireless communication applications usually operates at constant frequencies and requires correspondingly narrowband circuits. In contrast to this, the frequency may vary from zero (no rotation) to a given maximum speed of rotation in the case of a rotational angle sensor. This makes it additionally necessary to design the corresponding circuits to be broadband circuits. An individual mixer stage is consequently used to double the angular frequency obtained at the output of the mixer in comparison with the input signal. After one or more mixer stages, the signal can then be processed using the digital methods already described further above. Alternatively, it is also possible to directly use the analogue signals obtained at the mixer output to determine the angle of rotation.